JP2002-048719A, published by the Japan Patent Office in 2002, proposes a rear wheel suspension device for a bicycle using a gas spring type cushion unit.
The cushion unit comprises a cylinder filled with gas, a piston enclosed in the cylinder, and a piston rod connected to the piston and projecting from the cylinder. The piston rod is connected to a frame of the bicycle and the cylinder is connected to a rear wheel axle.
The piston delimits two operation chambers in the cylinder. These chambers are connected to each other via a valve provided outside the cylinder. The valve has a section which connects the chambers directly and a section which connects the cylinders via a check valve. The check valve shuts off a gas flow generated when the piston rod invades the cylinder while allowing a gas flow generated when the piston rod projects from the cylinder.
When the check valve shuts off the gas flow, gas in the chamber beneath the piston can no longer escape, and therefore contracts within the chamber. As a result, the spring constant of the gas spring against contraction of the piston rod becomes greater than in the case where the check valve does not shut off the gas flow.
When pedals are depressed strongly to start the bicycle, or in other words pedaling is performed to start the bicycle, squatting of the bicycle, a phenomenon in which the bicycle frame is pushed down onto the rear wheel due to a temporal increase in the load on the rear wheel, occurs. When squatting occurs, a large compression load acts on the gas spring. By causing the valve to shut off the gas flow generated by contraction of the cushion unit, the spring constant of the cushion unit is increased and the sinking of the bicycle frame onto the rear wheel is suppressed to be small. In contrast, when the bicycle is running normally, the valve is operated to interconnect the two chambers, thereby ensuring riding comfort under a smaller spring constant.